This invention relates to the field of spectroscopy, and particularly to spectrometers of the Fourier transform type. Such spectrometers generally incorporate a "division of amplitude" (Michelson) interferometer in which the source radiation is divided into two "arms" which are separately reflected and then recombined.
More specifically, the present invention is concerned with "dual beam" spectrometers having the same general purpose as those disclosed and claimed in Doyle U.S. Pat. No. 4,183,669 issued Jan. 15, 1980. As explained in that patent, the term "dual beam" refers to the fact that two distinct optical paths are used in the spectrometer, in order to "simultaneously obtain (a) data from a material sample under study and (b) data for reference purposes from a sample-free region".
As explained in U.S. Pat. No. 4,183,669 "the importance of dual beam Fourier spectroscopy stems from the fact that the interferogram corresponding to a broad radiation spectrum will have a very high peak value (central maximum) when the path lengths of the two interferometer arms are equal. In order to electronically perform the Fourier transformation required to obtain the spectrum, it is first necessary to digitize the interferogram, and the cost of analog-to-digital (A/D) converters rises rapidly with increased resolution. The resolution required of the input A/D converter will normally be much greater than that of the resulting spectrum".
Whereas the art prior to U.S. Pat. No. 4,183,669 disclosed a dual beam approach in which an "optical subtraction" method was used to reduce the central maximum of the interferogram, that patent disclosed a dual beam approach using an "electrical subtraction" method for producing a similar result.
Certain performance limitations have become apparent in the dual beam Fourier spectrometer of U.S. Pat. No. 4,183,669, which the present invention is intended to overcome. In the disclosure of that patent, separation between two optical beams is accomplished by allowing the beams to propagate at slightly different angles through the Michelson interferometer. When those beams are brought to a focus, they are spatially separated and hence can be directed along different optical paths to different detectors.
The foregoing arrangement has exhibited the following deficiencies:
(1) Because the two beams are propagated from slightly different areas of the infrared (IR) source, they may have slightly different spectral characteristics or may vary with time relative to each other; and
(2) Since the beams propagate at different angles, they may experience slightly different optical path length variations as the interferometer is scanned. This will lead to incomplete cancellation of the two interferograms and to spurious artifacts in the differential spectrum. To avoid this problem, the beam directions have to be adjusted so as to make equal and opposite angles with the interferometer axis (i.e. scanning direction) so that the angular effects are the same. This adjustment may require very careful manipulation.